State and explain two adaptations of a red blood cell?

Red blood cells have a biconcave disk shape, meaning that it increases their surface area and allows for more rapid diffusion of oxygen into and out of the cell.

Red blood cells do not have a nucleus so they can contain more haemoglobin, this is what combines with oxygen to form oxyhaemoglobin and so the fact that red blood cells do not contain a nucleus means that they can maximise their oxygen carrying capacity.